1. Field of the Invention
The present invention relates to a lithographic technique used in the semiconductor industry and the like and, more particularly, to a phase shifting mask which can realize a high resolution and a method of forming a pattern.
2. Description of the Related Art
In the semiconductor industry, with an increase in the integration density of ICs, it is required to manufacture fine patterns. In order to realize this, the resolution in lithography must be increased. As a means for achieving a high resolution, phase shifting methods have been proposed. As such phase shifting methods, various methods including a Levenson method (MARC D. LEVENSON: IEEE TRANSACTIONS ON ELECTRON DEVICE VOL ED-29 NO. 12 DECEMBER (1982) 1828) are known. Transmission masks are generally used in these methods.
A technique of using an exposure mask as a transmission phase shifting mask has been proposed. This mask is obtained by forming translucent and transparent regions on a transparent substrate such that the phase difference between light beams respectively transmitted through the translucent and the transparent regions becomes substantially 180.degree.. An example of this exposure mask is disclosed in U.S. Pat. No. 4,890,309 to Smith et al. of MIT. In this mask, a multilayered structure is employed to realize the optimal values of a film transmissivity characterizing a translucent region and a phase difference with respect to light transmitted through a transparent region.
In the future, with an increase in the integration density of LSIs, in order to achieve a technique of forming finer patterns, the wavelength of light emitted from a light source must be decreased. Especially for a 1-Gbit DRAM, a 0.2-.mu.m pattern is required; and for a 4-Gbit DRAM, a 0.1-.mu.m pattern. In order to realize these patterns, ArF (193 nm) or exposure light having a shorter wavelength must be used.
However, a currently used transmission substrate such as a quartz substrate also absorbs light having a short wavelength. For this reason, variations in transmissivity due to light absorption and a deterioration in durability due to thermal expansion raise problems. In order to solve these problems, searches for materials for transmission phase shifting masks, which absorb no light, have been made, or attempts have been made to change the optical characteristics of the masks.
A resolution in lithography is in direct proportion to a numerical aperture and in inverse proportion to a wavelength. Since an increase in numerical aperture is limited, the wavelength of light needs to be decreased to increase the resolution. However, as the wavelength of light emitted from an exposure light source decreases, a conventional transmission mask undergoes a decrease in transmissivity, resulting in a decrease in resolution. For this reason, for a transmission phase shifting mask, a material which can obtain a high transmissivity with respect to light having a short wavelength must be developed. It is, however, difficult to find or manufacture a material which can obtain a high transmissivity with respect to light having a short wavelength.
Under the circumstances, a reflection phase shifting mask has recently been proposed, which is free from the problem of a deterioration in quality due to radiation of light in a transmission phase shifting mask. In this mask, a recess portion (phase shifting portion) is formed in a substrate surface, and a phase difference of 180.degree. is realized by using an optical path difference between the bottom surface of the recess portion and the substrate surface.
The following problem is posed in a mask of this type. The recess portion is formed by etching. However, it is very difficult to accurately control the etching depth. Especially, it is inevitable that the etching depth varies depending on the coarseness/fineness of patterns. When reflection of light is utilized, the amount of change in optical path length is equivalent to twice the amount of change in etching depth. It is, therefore, substantially difficult to realize a phase difference of 180.degree. between light reflected by the substrate surface and light reflected by the phase shifting portion.
As a method of improving a transmission phase shifting mask, a method of using a mask blank including a single-layered translucent film which can arbitrarily change the complex index of refraction in a given region has been proposed by the present inventors (Japanese Patent Application No. 4-327623). As a material for this single-layered translucent film, a thin silicon nitride film formed by reactive sputtering is available. By exposing this film using a mask with patterns such as holes and lines, the resolution and the depth of focus can be increased.
In a method of this type, however, the following problem has been posed. When a material having a large refractive index is used for a translucent region, the reflectance at the boundary surface between the translucent film and an exposure atmosphere becomes high. Light reflected by a film surface is focused, as an inverted mask pattern, on a wafer via a demagnifying lens. For this reason, a region which should be a dark portion has a light intensity, so that deteriorations in resist pattern shape such as film thinning are caused.